A commercially available solid electrolyte capacitor as a rule comprises a porous metal electrode, an oxide layer on the metal surface, an electrically conductive solid which is incorporated into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation.
Examples of solid electrolyte capacitors are tantalum, aluminium, niobium and niobium oxide capacitors with charge transfer complexes, or pyrolusite or polymer solid electrolytes. The use of porous bodies has the advantage that because of the high surface area a very high capacitance density, i.e. a high electrical capacitance over a small space, can be achieved.
π-Conjugated polymers are particularly suitable as solid electrolytes because of their high electrical conductivity. π-Conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals in respect of processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene which is used industrially being poly-3,4-(ethylene-1,2-dioxy)thiophene, often also called poly(3,4-ethylenedioxytliophene), since it has a very high conductivity in its oxidized form.
Technical development in electronics increasingly requires solid electrolyte capacitors having very low equivalent series resistances (ESR). Reasons for this are, for example, falling logic voltages, a higher integration density and increasing cycle frequencies in integrated circuits. Furthermore, a low ESR also lowers energy consumption, which is particularly advantageous for mobile battery-operated uses. There is therefore the desire to reduce the ESR of solid electrolyte capacitors to as low a value as possible.
European Patent Specification EP-A-340 512 describes the preparation of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymer, prepared by oxidative polymerization, as a solid electrolyte in electrolyte capacitors. Poly(3,4-ethylenedioxythiophene), as a substitute for manganese dioxide or charge transfer complexes in solid electrolyte capacitors, lowers the equivalent series resistance of the capacitor due to the higher electrical conductivity, and improves the frequency properties.
In addition to a low ESR, modern solid electrolyte capacitors require a low residual current and a good stability towards external stresses. During the production process in particular, high mechanical stresses arise during the encapsulation of the capacitor anodes, which can greatly increase the residual current of the capacitor anode.
Stability towards such stresses and therefore a low residual current can be achieved above all by an approx. 5-50 μm thick outer layer of conductive polymers on the capacitor anode. Such a layer serves as a mechanical buffer between the capacitor anode and the contacting on the cathode side. This prevents e.g. the silver layer (contacting) from coming into direct contact with the dielectric or damaging this under mechanical stress, and the residual current of the capacitor from increasing as a result. The conductive polymeric outer layer itself should have so-called self-healing properties: relatively minor defects in the dielectric on the outer anode surface which arise in spite of the buffer action are electrically insulated in that the conductivity of the outer layer at the defect is destroyed by the electric current.
The formation of a thick polymeric outer layer by means of an in situ polymerization is very difficult. In this context, the layer formation requires very many coating cycles. Due to the high number of coating cycles, the outer layer becomes very inhomogeneous, in particular the edges of the capacitor anode are often inadequately covered. Japanese Patent Application JP-A 2003-188052 reports that a homogeneous covering of the edges requires expensive coordination of the process parameters. However, this makes the production process very susceptible to malfunctions. An addition of binder materials for faster building up of layers is also difficult, since the binder materials impede the oxidative in situ polymerization. Furthermore, the layer polymerized in situ as a rule has to be freed from residual salts by washing, as a result of which holes arise in the polymer layer.
A dense electrically conductive outer layer with good covering of the edges can be achieved by electrochemical polymerization. However, electrochemical polymerization requires initial deposition of a conductive film on the insulating oxide layer of the capacitor anode and then electrical contacting of this layer for each individual capacitor. This contacting is very expensive in mass production and can damage the oxide layer.
The use of formulations which comprise the powder of a conductive polymer and binder have, because of high contact resistances between the individual powder particles, too high an electrical resistance to render possible production of solid electrolyte capacitors having a low ESR.
In Japanese Patent Applications JP-A 2001-102255 and JP-A 2001-060535, a layer of polyethylenedioxythiophene/polystyrenesulfonic acid (PEDT/PSS), also called polyethylenedioxythiophene/polystyrenesulfonic acid complex or PEDT/PSS complex, is applied directly to the oxide film for protection of the oxide film and better adhesion of the solid electrolyte to the oxide film. The outer layer is then applied to this layer by means of in situ polymerization or by impregnation of the capacitor anode with tetracyanoquinodimethane salt solution. However, this method has the disadvantage that the PEDT/PSS complex does not penetrate into porous anode bodies having small pores. As a result, modern, highly porous anode materials cannot be used.
U.S. Pat. No. 6,001,281 describes, in the examples, capacitors having a solid electrolyte of polyethylenedioxythiophene (PEDT) prepared in situ and an outer layer of PEDT/PSS complex. A disadvantage of these capacitors, however, is that they have a high ESR of 130 mΩ and higher.
In the not yet published German Patent Application DE-A-10349112, a polymeric outer layer is produced by application of a dispersion comprising at least one polymeric anion and at least one optionally substituted polyaniline and/or at least one polythiophene having recurring units of the general formula (I), (II) or recurring units of the general formula (I) and (II)
and a binder. Although the covering of the edges can be improved by this process, dense polymeric outer layers cannot be reproduced reliably, however, by this means.
There therefore continues to be a need for an improved process for the production of solid electrolyte capacitors having a low equivalent series resistance (ESR), with which a dense polymeric outer layer can be realized simply and reliably reproduced with good covering of the edges. The object was therefore to provide such a process and the capacitors improved by this means.